The present invention relates generally to a servo system in a disc drive. More particularly, the present invention relates to implementing a robust servo control system in a disc drive.
A typical disc drive includes one or more discs mounted for rotation on a hub or spindle. A typical disc drive also includes one or more transducers supported by a hydrodynamic air bearing which flies above each disc. The transducers and the hydrodynamic air bearing are collectively referred to as a data head. A drive controller is conventionally used for controlling the disc drive system based on commands received from a host system. The drive controller controls the disc drive to retrieve information from the discs and to store information on the discs.
An actuator operates within a negative feedback, closed-loop servo system. The actuator typically includes an actuator arm that supports a flexure or flexure assembly which, in turn, supports the data head. The actuator moves the data head radially over the disc surface for track seek operations and holds the transducer directly over a track on the disc surface for track following operations.
Information is typically stored on the discs by providing a write signal to the data head to encode information on the surface of the disc representing the data to be stored. In retrieving data from the disc, the drive controller controls the actuator so that the data head flies above the disc, sensing the information on the disc, and generating a read signal based on that information. The read signal is then decoded by the drive controller to recover the data represented by the information stored on a disc, and consequently represented in the read signal provided by the data head.
Accurate positioning of the data head over a track on the disc is of great importance in writing data to the disc and reading data from the disc.
In prior systems, servo operations were accomplished based on a dedicated servo head. In a dedicated servo type of system, servo information is all written to one dedicated surface of a disc in the disc drive. All of the heads in the disc drive are mechanically coupled to the servo head which is used to access the servo information. Thus, all of the heads in the dedicated servo disc drive are positioned based on the servo information read from the servo surface. This type of system allows the disc drive to conveniently execute parallel read and write operations. In other words, with appropriate circuitry in the drive controller, read and write operations can be executed in parallel using a plurality of the data heads mounted on the actuator, the data heads being simultaneously positioned based on the servo information read from the dedicated servo surface.
However, track densities on magnetic discs have been increasing for many years. Increased track densities on the magnetic disc require more accurate and higher resolution positioning. The mechanical offset between heads in a dedicated servo system can exceed one track width. Thus, the industry has seen a shift to embedded servo information in certain applications.
In an embedded servo system, servo information is embedded on each track on each surface of every disc. Thus, each data head returns a position signal independently of the other data heads. Therefore, the servo actuator is used to position each individual data head while that particular data head is accessing information on the disc surface. The positioning is accomplished using the embedded servo data for the track over which the data head is then flying.
While this results in increased positioning accuracy and higher resolution in the positioning process (because the data head is positioned independently of any other data heads), certain disadvantages are encountered because of increased track density and mechanics. One of the disadvantages is that in typical embedded servo systems, the ability to execute parallel read and write operations using a plurality of heads is lost. That is because the servo system is positioning based on information received by one individual data head, and the mechanical tolerances are inadequate to accurately position the other data heads in high track density systems. Also, current actuators are incapable of independently positioning the data heads. Thus, embedded servo systems, to date, have been unable to execute parallel read and write operations, such as simultaneously reading or writing a full cylinder in a disc drive.
Because of these differences between dedicated and embedded servo systems, there is a trade off between servo sample rate and efficient user data storage.
In addition, there are many issues in both systems which affect the positioning accuracy and precision of the servo system. Some of the most important issues include the following:
1. Servo sample rate. The sample rate is limited by the spindle speed and the number of servo sectors per track in an embedded servo system.
2. Structural modes in the arm and head suspension.
3. External shock and vibration, which can be either linear or rotational or both.
4. Written-in position error which results from tracking errors which occurred when the servo tracks were written. This results in repeatable runout. Runout refers to the total positioning error while executing a track following operation. Runout is typically referred to as a static deviation while tracking over long periods of time. Because written-in position error is synchronized to the spindle speed, it is referred to as repeatable runout.
5. Bearing non-linearities. Such non-linearities result in rotational drag and hysteresis, particularly when the actuator is moving at low velocities.
6. Non-linearity due to flex circuit bias forces on the actuator. In other words, the actuator is coupled to the disc drive controller through a flex circuit. When the actuator positions the transducer at different radial positions on the disc, the flex circuit bias forces on the actuator can change.
7. Disc flutter which results in non-repeatable runout. The amount of disc flutter is dependent on the spindle speed and the stiffness of disc substrate.
8. Gain variation resulting from magnetic transducer non-linearity cross-track.
9. Position error sample noise resulting from media magnetic variability, electronic noise, etc.
Prior conventional servo controllers have included proportional-integral-derivative (PID) controllers which are composed of two components: an observer and a regulator. The observer receives input position information each time a servo sector is crossed, and estimates position and velocity. The regulator then provides feedback on the observed signals. In a seek mode, the regulator typically zeros the error between a reference velocity trajectory and the observed velocity. In track following mode, the regulator zeros the error between the desired track position and the observed track position. The regulator controls according to a PID control technique. Suffice it to say that PID controllers are highly advantageous in many disc drive applications.
However, PID controllers may not be advantageous or desirable in all disc drive applications. For example, it may be desirable to provide microactuators between the flexure assembly and the transducer or slider assembly, or on the actuator arm, or on the suspension or flexure assembly. Where microactuators are provided, the servo actuator system evolves from a single input single output (SISO) system where the input is an error signal and the output is a voice coil current signal, to a multiple input multiple output (MIMO) system which receives a variety of inputs from the microactuators and provides a position output signal to the voice coil motor and each of the microactuators. While such a system could be controlled by simply decentralizing a PID controller, this may present problems. For example, if a plurality of heads are simultaneously positioned, the positioning of one data head can be affected by the simultaneous positioning of adjacent or proximate other data heads. Further, high bandwidth positioning can excite the structural modes of the drive and cause vibration, ringing, or other interference which can tend to interfere with the positioning of adjacent data heads.
Many of these issues are mitigated by employing a general discrete time output feedback control approach. However, a number of problems present themselves when attempting to implement a discrete-time system on a fixed-point digital signal processor in a disc drive. For example, the computation capability provided in digital signal processors which may be used in a disc drive is typically quite limited. This raises a number of significant issues. The size and number of registers in a digital signal processor can be quite limited. When performing matrix calculations, the number of bits required to store the results of intermediate calculations may exceed the capacity of the registers in the digital signal processor. Thus, overflow presents a significant hurdle. In addition, the computation speed and structure, as well as the memory capacity, in a digital signal processor can make some matrix calculations highly impractical, simply due to the number of calculations which must be performed. Also, substantially all digital signal processors are fixed-point processors. Thus, implementing a linear discrete-time system in a digital signal processor can be highly impractical. Further, quantization errors in a conventional DSP can become appreciable rendering the control accuracy of the DSP in a servo system in the disc drive impractical.
The present invention addresses at least some of these or other problems, and offers advantages over the prior art.
A method of implementing a discrete-time servo controller in a fixed-point digital signal processor (DSP) for controlling a servo actuator in a disc drive includes obtaining a stable, closed-loop representation of the servo controller and the servo actuator. Bounds on the controller states are derived from the stable closed-loop representation. The controller states are scaled based on the controller state bounds, and the servo controller is configured in the DSP based on the scaled controller states.
The present invention is also directed to a disc drive which includes a servo controller coupled to a servo actuator used for controlling actuation of the servo actuator. The servo controller is configured with scaled controller states, scaled based on a bound on a state evolution for a closed-loop system including a servo controller and the servo actuator.
These and other beneficial features of the present invention will become apparent upon review of the following figures and related explanations.